This invention relates to controls for thermal printers, and particularly to controlling the drive current supplied to heating elements of a thermal printer.
Thermal printers employ thermal energy to form images on a media. Broadly, such printers operate by either applying thermal energy to the media to alter image characteristics of the media, or by thermally energizing a hot melt wax ink ribbon to transfer ink to the media. These printers are characterized by stationary heads that extend across the width of the media and have heating elements for each pixel location on the media. The number of heating elements is dependant on the resolution of the printer (number of dots per inch, or dpi, along each line of print) and the width of the printer carriage. Thermal printers usually have a large number of heating elements, often numbered in the thousands.
The picture elements (pixels) formed on the media may be binary elements (full tone or no tone) whose size is, in part, dependent upon the amount of heat applied by the corresponding heating element. Alternatively, the pixels formed on the media may be contone elements (gradation over a range from no tone to full tone) whose intensity is, in part, dependent upon the amount of heat applied by the corresponding heating element. In either case, the amount of heat applied by a heating element is, in part, dependent upon the amount of drive current supplied to the heating element, the resistance value of the heating element, the ambient temperature of that heating element at the start of the current cycle, and the temperature of neighboring heating elements. Most thermal printers employ controls to overcome or at least reduce the effects due to heat generated by neighboring heating elements.
In U.S. Pat. No. 5,519,426 issued May 21, 1996 and co-pending application Ser. No. 08/298,936, filed Aug. 31, 1994 for "Method and Apparatus for Controlling a Thermal Print Head" by Lawrence J. Lukis, J. Mark Gilbert and Danny J. Vatland and assigned to the same assignee as the present invention, there is described a technique for developing thermal image data to control internal switches to the heating elements of a thermal print head to thereby control the application of drive current to the thermal print head. The Lukis et al. application describes the generation of drive current signals for the heating elements of a head to generate a requisite binary image size by the selected heating element, while taking into account the ambient temperature of the heating elements and thermal interaction between adjacent heating elements. In a preferred form of the invention described in the Lukis et al. application, the drive currents are right or end justified so that they terminate simultaneously, but the start times for the current pulses vary depending on the length of time that the particular element is to be energized.
It is known that tolerances in the manufacture of thermal print heads (including those for thermally driven ink jet printers) result in resistance values of the elements that vary as much as .+-.15%. Moreover, in use, the heating elements may thermally degrade, thereby altering (usually increasing) the resistance values of the heating elements. Thus, the range of resistance values of the heating elements may be 30% or more, depending on manufacturing tolerances and the manner in which the various heating elements degrade. Thermal degradation usually increases the resistance value so that the heating element generates less heat for a given quantity of applied voltage, thereby deteriorating the quality of the print image and shortening head life. Thus, the print head may generate dots that are smaller (in binary printing) or less intense (in contone printing) than intended. Consequently, it is common to employ compensation techniques to compensate for differences in resistance values, particularly due to manufacturing tolerances.
The resistances of the heating elements are usually measured as a function of the load current of the elements. However, leakage current in the thermal head interferes with accurate measurement of the load current of a given heating element, thereby making resistance measurement inaccurate.